The present invention pertains to a sewing machine with a needle bar driven in an upwardly and downwardly movable manner with a needle feed mechanism, a feed dog and the needle driven oscillatingly in the direction of feed, a stitch regulating device associated with the feed mechanism, and a device for controlling the sewing machine and for approaching a predetermined end point of a seam at a spaced location from the edge of a workpiece, the device comprises a sensor, which is arranged in front of the needle and triggers the operation for positioning the needle at the seam end point during the passage of the workpiece edge, the device has a pulse generator determining the speed of rotation and the angular position of a machine shaft and a computer, which controls the number of residual seam stitches and, via an adjusting device, the setting of the stitch regulating device for reaching the seam end point as a function of the distance between the needle and the sensor and the angular position of the machine shaft, which is determined at the time of the edge recognition.
A means for approaching the end point of a seam, in which this operation is triggered by means of a sensor scanning the passage of the workpiece edge extending at right angles or at an angle to the seam, is known from DE 33 24 715 C2 (U.S. Pat. No. 4,528,923) in a sewing machine with lower and needle feed. The means comprises, furthermore, a pulse generator detecting the speed of rotation and the angular position of a machine shaft, by which the size of the partial stitch already pushed during the edge recognition is determined. The number of residual seam stitches, which are unchanged in length, and the length of the shortened end stitch or the length of a plurality of uniformly shortened residual seam stitches are calculated by means of a computer as a function of the distance between the middle position of the needle of the sewing machine and the sensor, the seam distance and the length of the partial stitch yet to be pushed.
To adjust the stitch regulating device accurately and also comparatively rapidly, the computer prompts the presetting of a stop for limiting the movement stop of a follower member coupled with the stitch regulating device. This follower member is moved very rapidly against the stop before the beginning of the one shortened end stitch or of the first of several shortened residual seam stitches by means of a compressed air cylinder and the stitch regulating device is thus adjusted to the desired shortened stitch length.
It is expressly emphasized in the DE-C2 that the adjustment of the stitch regulating device takes place during the short time available during the phase of standstill between the needle and the workpiece. This should be understood to mean that the exact adjustment of the stitch regulating device can take place only in the middle of the phase of feed of the feed dog and the needle and consequently with the lower needle feed.
The same situation that was described in the specification of the DE-C2 concerning the sewing unit for sewing calender envelopes of the firm of Adler would otherwise occur. A sewing machine with lower feed is used in this prior-art sewing unit. When the stitch regulating device is set to the stitch length zero in this unit during the feed motion in order to interrupt the feed motion of the workpiece as a result, this can be performed with sufficient accuracy only if the feed dog has performed exactly half of its feed motion during the adjustment of the stitch regulating device and is therefore in the middle of the needle. Because of the existing kinematics of stitch regulating devices, the feed dog always assumes the middle position during the adjustment of the stitch regulating device to zero. If the adjustment of the stitch regulating device is performed before or after the middle position of the feed dog, the latter will therefore perform a forwardly or backwardly directed offset movement toward the middle of the needle. Since this happens while a feed step is being performed, during which the feed dog is in contact with the workpiece, the workpiece also performs the offset movement of the feed dog, and this movement is therefore also called pushing movement.
Such an offset movement also occurs in the sewing machine with lower and needle feed known from the above-mentioned DE 33 24 715 C2 during the adjustment of the stitch regulating device. The effects of this on the stitch length of the sewing stitch to be performed thereafter are illustrated in the drawing on the basis of the movement of the needle, whose tip describes elliptical movement paths.
FIG. 4 shows the movement path of the needle 5 during a feed cycle, during which the stitch regulating device is set to the stitch length S and the needle 5 feeds the workpiece W together with the feed dog 9 by the amount of the set stitch length S in the direction of feed V. The movement path of the needle 5 is now symmetrical to the middle of the needle Nm.
FIG. 5 shows the situation that would occur if the stitch length Sb of a residual seam end stitch located at the end point of the seam were detected by means of the computer after an edge recognition and the stitch regulating device were set to the stitch length Sb during the return phase of the needle and the feed dog. After the completion of the last feed cycle, the last point of insertion of the needle 5 or the point at which the needle exits is located at a distance S/2 in the direction of feed V behind the middle of the needle Nm. If the stitch regulating device is set to the lower value Sb, the needle bar first performs an offset movement in the direction of the middle of the needle because of the above-mentioned existing kinematics of stitch regulating devices. The needle bar then moves in front of the middle of the needle Nm by the amount Sb/2 when viewed in the direction of feed V. A stitch length of Sz=S/2+Sb/2 is thus obtained for the next sewing stitch. If, e.g., the stitch length is S=4 mm and a stitch length of Sb=1 mm was calculated for the residual seam end stitch, a stitch length of Sz=4/2+1/2=2.5 mm would be obtained for the last stitch. The last stitch would thus be too long by 1.5 mm.
To avoid such an offset movement leading to an unusable result, the adjusting movement of the stitch regulating device must be, as was mentioned, very rapid, i.e., sudden in the middle of the feed phase. However, the consequence of this is that the adjustment operation causes a vibration of the sewing machine and a corresponding noise. If, by contrast, the adjustment operation shall take place more slowly, the sewing machine would have to be briefly stopped for this purpose, which would interrupt the sewing operation and lead to a loss of time.
These problems are avoided in the means known from DE 33 42 391 C1 (U.S. Pat. No. 4,587,915) such that the setting of the stitch regulating device remains unchanged in this means for the performance of a single shortened end stitch or a plurality of shortened residual seam stitches, and the feed dog is raised, instead, during its return phase, which normally takes place in the lowered position, until it pushes the workpiece back against the normal direction of feed by the amount by which the next end stitch or residual seam stitch shall be shortened. Aside from the fact that this means is technically highly complicated, another drawback lies in the fact that it may sometimes lead to inaccurate results because the feed dog, which now acts as the only feed means, is operating against the normal direction of feed of its teeth during the reverse push, and a slip, whose amount depends very strongly on the properties of the particular fabric of the workpiece, may therefore develop between the feed dog and the workpiece.
The basic object of the present invention is to provide a sewing machine with a means for approaching a predetermined end point of a seam, which has a simple design and in which the adjustment of the stitch regulating device can be carried out without interruption of the sewing operation.
The present invention is based essentially on the idea of adjusting the stitch regulating device during the phase of return of the feed means before the sewing of a shortened end stitch or residual seam stitch such that the offset path of the feed means, which thus arises, is also taken into account, so that the end stitch of the seam is carried out with the necessary stitch length and the seam is terminated precisely at the point predetermined by the amount of the particular seam distance.
The process is also suitable for sewing uniformly shortened residual seam stitches. However, the particular offset path, which is consequently the current offset path, must be taken into account in this case for sewing every individual residual seam stitch, and the stitch regulating device must be set separately for every individual residual seam stitch.
The law that is equally applicable to both types of seam end management, by which the amount of the offset path is taken into account without having to determine it directly, is described.
The adjusting device for the stitch regulating device is formed by a stepping motor. This design of the adjusting means, which is simple compared with the state of the art, is made possible by the fact that by taking into account the offset path of the feed means, which arises during the adjustment of the stitch regulating device, the adjustment operation does not need to take place abruptly, but it can be performed during the comparatively long time of the return phase of the feed means.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.